We've long said that cutting meat and dairy out of your diet, or at minimum cutting back deeply on their consumption, is one of the most powerful personal steps you can take towards mitigating climate change. But new analysis from Worldwatch Institute shows that the impact of raising livestock and poultry is much greater than previously thought and actually amounts to approximately 51% of human-caused greenhouse gas emissions:The FAO's widely-cited 2006 report Livestock's Long Shadow listed annual greenhouse gas emissions from livestock to be 11.8%. However, Worldwatch shows that the FAO severely undercounted or misallocated emissions from a number of areas in the livestock production chain.

More than anything it seems an exercise in making sure emissions are properly attributed.

photo: Mike Rosenberg via flickr.Livestock Respiration = +13.7%Though the FAO considers respiration by livestock to not be a net source of CO2 (anything emitted by the animals is naturally offset by other natural processes) and that livestock could actually be considered a carbon sink, Worldwatch report authors Robert Goodland and Jeff Anhang believe this to be flawed logic.

Goodland and Anhang contend, "the amount of carbon stored in livestock is trivial compared to the amount stored in forest cleared to create space for growing feed and grazing livestock."

They go on to argue that in one crucial way livestock are like automobiles: Both are essentially human inventions, for our "convenience" but not survival. We measure the direct emissions of latter but not the former and "by keeping GHGs attributable to livestock respiration off GHG balance sheets, it is predicatble that they will not be managed and their amount will increase -- as is in fact happening."

Overlooked Land Use = +4.2%It should now be a familiar story to TreeHugger readers, land being converted from tropical forest (a huge carbon sink) to cattle grazing or agricultural lands (a comparatively tiny carbon sink). And this is accounted for in the FAO analysis, but Worldwatch argues that what needs to also be considered is the forgone carbon sequestration by using 26% of land worldwide for cattle grazing and 33% of arable land for growing food for livestock.

The impact of letting this land regrow as forest, or even used for growing food for humans or sustainably-produced biofuels, is such that a bit over 4% of global greenhouse emissions ought to be tallied up under this column.

Methane = +7.9%Of all human-induced methane emissions, 37% comes from livestock. That much isn't in debate. But according to Goodland and Anhang the global warming potential of methane used by the FAO is too low.

Due to the fact that methane has greater warming potential than CO2, though dissipates much faster, rather than use a 100-year timeframe for calculating the impact as is commonly done, a 20-year timeframe is more appropriate, as is supported by the IPCC.

Using this methodology, rather than methane emissions from livestock totaling 3.7% of the global amount, they would be 11.6% -- an increase of 7.9%.

4% from the fact that the FAO report cited slightly older stats for the amount of livestock products produced -- from 2002 to 2009 the tonnage of livestock products increased 12%, which was unaccounted for.

4.7% from undercounting of the amount of livestock in the world, their emissions, and overgeneralization of production efficiency -- the report authors contend that while the FAO cites much data from conditions in Minnesota, its erroneous to extrapolate these out to the world, as these conditions are more efficient than are generally found globally.

4.7% from aspects of the livestock production chain, and byproducts of it such as the leather and fur industries, as well as packaging and waste disposal, which wouldn't otherwise exist. Also included in this percentage is treatment of illnesses related to meat-intensive diets and zoonotic illnesses (i.e. swine flu).

The Solution = Widespread Adoption of Vegetarian DietsThe original report goes on for several pages about the economic opportunities that exist for food companies and how best to bring about industry changes, as well as how to market vegetarian foods, with a strong emphasis on processed meat substitutes such as soy- and seitan-based fake meats (which seems a bit heavy-handed... how about legumes and whole grains??).

But the overall message can really be boiled down to this: One of the cheapest and most effective ways of mitigating climate change is eating far less meat and dairy or (better yet) eating none and adopting a full vegetarian or vegan diet. It's going to take cultural shift to do this, to be sure, but the effect is huge.

Some statistical backing for that: A Dutch report, which came out about six months ago, showed that if enough people adopted a vegetarian diet we could reduce the costs of mitigating climate change by up to 70% -- even if people returned to eating meat at levels normal for our grandparents, those costs could be reduced by 50%.